The present invention and embodiments thereof is directed to a method and filter for the reduction of noise in radiography and, more particularly in fluoroscopic images. The invention and embodiments thereof is directed to space-time filtering of noise in images. The field of the invention is more particularly directed to the reduction of noise in images acquired in temporal sequences.
The prior art scanner type image acquisition apparatuses can be used to obtain images of an object, such as the interior of a living organism, and particularly the interior of a human body. Such apparatuses can be used to obtain images of internal organs and perform diagnoses. These images are obtained by the exposure of the object, such as a patient, to radiation that is received on a detector after it has gone through the object. The detector then produces an image that can be interpreted by a specialist practitioner. These images contain noise known as fluoroscopic noise. This fluoroscopic noise is the resultant of quantum noise, caused by the nature of the radiation, and electronic noise, caused by the nature of the detector.
Fluoroscopic noise is present in the image and therefore causes interference with the signal, known as the useful signal or information signal, present in the image. This makes the interpretation of the images difficult, uncertain or even impossible.
In the prior art, there are known ways of improving the signal-to-noise ratio, or SNR. In order to increase the ratio of information present in the image, it is possible to increase the intensity of the radiation. However, this makes the examination more traumatic for the patient, something that is unacceptable for health reasons.
In the prior art, there is a known technique of filtering fluoroscopic noise. In this technique, a temporal mean is taken between the value of two points, or pixels, having the same coordinates in two images. The two images belong to one and the same sequence of images representing one and the same region. The two images have the same framing and parameters of exposure but are taken at different times. If an image I is filtered at the time t, then, for each pixel of the image I, a mean is taken with the corresponding pixel of the image I′ obtained at t−1.
This method has several drawbacks. A first drawback lies in the low reduction of noise in the filtered image. This reduction is of the order √2 at most. A second drawback lies in the problem of remanence, or the appearance of phantoms in the filtered images. Consider the acquisition of images of an artery into which it is known that a guide has been inserted. A guide is a cylindrical metal object that is introduced into the artery and is therefore visible in radiography. Given that the guide moves at high speed, it is possible that it will be present in the imaged zone at the time t−1, but not at the time t. However, since the image filtered at the time t is obtained by taking the average of the image acquired at the time t and the image acquired at the time t−1, a filtered image is obtained for the time t, and this filtered image shows a guide which, nevertheless, was not present at this time. Thus, an erroneous piece of information has been added to the image of the time t.
A prior art solution gives a noise reduction of 15% and a remanence rate of 10%. This is unsatisfactory because it is a source of error or makes it impossible to carry out diagnosis.